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Tuesday, September 30, 2008

NY Times [italicized extract below] — On the findings that Sarah Tishkoff and her team found, as have others too, showing that the C−13.9kbT allele which is determined to be a predictor of lactose tolerance in Europe, has not been effective in predicting lactose tolerance in Africa, even though this is a continent where lactose tolerance amongst various groups, particularly those that practice pastoralism, has been well documented. The reason for this phenomenon is 'convergent evolution', wherein several distinct alleles different from the European examples appear to be promoters of lactose tolerance...

But it seemed to be largely absent from Africa, even though pastoral peoples there generally have some degree of tolerance...

After testing for lactose tolerance and genetic makeup among 43 ethnic groups in East Africa, she and her colleagues have found three **new** mutations **all independent of one another** and **of the European mutation**, that keep the lactase gene permanently switched on.

—1) Lactase persistence varies widely in frequency among different human populations, both betweenand within continents.

Examples of within continents variations...

*E.g. 1 - in Europe itself variation is apparent - Almost all Dutch people and 99 percent of Swedes are lactose tolerant, but the mutation becomes **progressively** less common in Europeans who live at increasing distances from the ancient Funnel Beaker region - NY Times, an article on Sarah Tishkoff's 2004 findings on lactase phenotypes in African samples.

*E.g 3 - After testing for lactose tolerance and genetic makeup among 43 ethnic groups in East Africa, she and her colleagues have found three **new** mutations **all independent of one another

Example of between continents variation:

*E.g. - After testing for lactose tolerance and genetic makeup among 43 ethnic groups in East Africa, she and her colleagues have found three **new** mutations **all independent of one another** and **of the European mutation**, that keep the lactase gene permanently switched on.

—2) To date [as of 2003], there have been no reports of allele frequencies for the C−13.9kbT polymorphism in populations living in Africa.

[as of 2004] The frequency of −13.9kb*T type was low or zero in most of the African groups tested. In relation to this, we have the following examples:

*E.g. 1 - In the African populations, the −13.9kb*T allele was only found in a few individuals; all but one of these individuals were from Cameroon, [and these were in the main, ancestral or paraphyletic R1*-M173 carriers]

*E.g. 2 - It is noteworthy that −13.9kb*T was not found in East Africa at all, even though the data sets included many known pastoralists and groups with a high frequency of lactase persistence.

—3) Previous studies of African populations showed variation in the frequency of lactase persistence among population groups, as well as a complex pattern of distribution (reviewed in Flatz 1987; Holden and Mace 1997; Swallow and Hollox 2000).

African populations display multiple lifestyles, with milk-drinking and non–milk-drinking groups often living in close proximity, and have complex demographic histories. Some examples below:

* E.g. 1 - Pastoralists, such as the Fulbe in Nigeria, typically have higher frequencies of lactase persistence than nonpastoralists in the same country—for example, the Yoruba and Igbo

* E.g. 2 - The lactase-persistence phenotype is usually observed at low frequencies in Bantu- and Khoisan-speaking groups (<20%); Comparisons of the predicted frequencies of lactase persistence, deduced from the frequency of −13.9kb*TT and −13.9kb*CT genotypes, with the reported frequencies obtained from lactose-tolerance testing, showed these were significantly different in all of the African populations except the Fulbe and the Hausa.

—4) Why it is misleading to use reports for African American candidates as representative of lactose tolerance phenotypes amongst Africans:

*Although the −13.9kb*T allele frequency in Americans with African ancestry is consistent with their lactase-persistence frequency (Enattah et al. 2002), there is known to be substantial admixture between African Americans and European Americans (Parra et al. 1998)

So, African American examples of the C-13.9kbT alleles could represent introgression from European Americans, since as already noted above, at the time of Mulcare et al.’s study, “To date, there have been no reports of allele frequencies for the C−13.9kbT polymorphism in populations living in Africa” and Mulcare et al. themselves found that “the frequency of −13.9kb*T is too low to explain the observed frequency of lactase persistence.”

*In relation to point 3 at the top - Though African American gene pool are largely reflective of those of their ancestral African populations, it doesn’t represent the overall gene pool of Africa, and it may well have been subjected to some micro-evolutionary processes, however low, since the time of separation from source African populations, including — as noted above — bidirectional genetic introgressionwith populations that are otherwise rarely geographically proximate to African populations.

*African American lifestyles may not necessarily parallel those of their respective ancestral populations and other African populations.

And as already noted, but not the least…

—5) While the C−13.9kb*T allele has been proclaimed to be a predictor of lactose tolerance in Europeans [see Enattah and colleagues (2002)], Mulcare et al.’s results show that the −13.9kb*T allele cannot be causal of lactase persistence in most Africans [although it could possibly explain lactase persistence in some Cameroonians].

About Me

The internet is obviously an invaluable tool, but it can also work the other way—i.e. comes with certain risks, however far in between. Sparing oneself from potential identity theft or abuse warrants a good measure of anonymity. Hence, the turn towards a pseudonym. This site was not conceived with view to winning blogosphere "popularity contests". Attention-grabbing controversial posturing is out of character here, although it's recognized that many out there do have opinions on any given topic, and that topics which instill polarizing viewpoints may indeed crop up by chance. The overriding goal here is to build a one-stop referential spot for things African, and in doing so, hopefully ease off the flood of misinformation about the continent. Reader commentary isn't restricted, provided that it's done in a respectful insult-free tone. Violation thereof will be met with rejection without reservation. Hope the site proves to be informative to those who care to avail themselves of information gathered herein. Be sure to check the "Developments" link on the top of the page, to look for possible updates elsewhere in the blog!